In high pressure sodium (HPS) lamps, the arc tube is made from monocrystalline alumina (sapphire) or polycrystalline alumina (PCA). Gas-tight ceramic-to-metal seals between the discharge tube and a pair of niobium current inleads, which have tungsten cathodes affixed thereto, close the ends of the discharge tube. Niobium (which may include an addition of about 1% zirconium) is used as the inlead material because its coefficient of thermal expansion closely matches that of the alumina arc tube. Further, niobium is resistant to sodium at high temperatures and has a relatively high permeability for hydrogen, allowing hydrogen impurities in the arc tube to escape therefrom and to be sorbed by a getter in the outer bulb which surrounds the arc tube. The niobium current inlead can take the form of a wire (U.S. Pat. No. 4,538,091) or a tube (U.S. Pat. Nos. 4,559,473; 5,026,311; 5,424,608) on which the tungsten electrodes are fixed, usually by crimping and/or welding or by brazing, usually with titanium.
A critical feature of HIPS lamps is the arc length, defined as the interior distance between electrode tips within the arc tube. In order to control the position of electrodes inside an arc tube, a positioning feature is provided on the niobium inleads. This positioning feature can be welded fine wires on the inlead, such as are shown on the above-mentioned U.S. Pat. No. 5,026,311; wire clips attached frictionally, such as shown in U.S. Pat. No. 4,538,091; deformations formed on the inlead, such as shown in U.S. Pat. Nos. 4,559,473 and 4,937,494; or specially shaped, cooperative apertures formed in the end seals of the arc tubes such as shown in U.S. Pat. No. 5,424,608. The location of the positioning feature is important in determining the backspace, the backspace being the distance between the upper or lamp side surface of the positioning feature and the top of the cathode coil. This spacing determines the arc length. Whatever feature has been employed in the past, if the inlead is to be useable on multiple lamp types, it has been necessary that it be formed at a different position on the inlead. This operation can increase the cost of manufacturing lamps and can lead to errors in electrode selection.